A dual window or bi-optical, point-of-transaction workstation of the type disclosed in U.S. Pat. No. 7,191,947 has been used to electro-optically read one-dimensional bar code symbols, each having a row of bars and spaces spaced apart along one direction, particularly of the Universal Product Code (UPC) type, stacked symbols such as Code 49 that introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol as described in U.S. Pat. No. 4,794,239, two-dimensional symbols such as PDF417 that increased the amount of data that could be represented or stored on a given amount of surface area as described in U.S. Pat. No. 5,304,786, on products in supermarkets, warehouse clubs, department stores, and other kinds of retailers for many years. A single, horizontal window is set flush with, and built into, a horizontal countertop of the workstation. A generally upright window is oriented generally perpendicularly to the horizontal window, or is slightly rearwardly or forwardly inclined relative to the countertop, and faces an operator at the workstation.
Behind each window is an electro-optical reader, e.g., a laser scan pattern generator that projects a scan pattern of scan lines through the respective window and detects light returning from the symbol through the window, or a solid-state imager that has a one- or two-dimensional sensor array of pixels, cells or photosensors that capture light returning from the symbol through the respective window. The photosensors correspond to image elements or pixels in the field of view (FOV) of the imager. Such an imager may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, as well as imaging optics and associated circuits for producing electronic signals corresponding to the one- or two-dimensional array of pixel information over the field of view.
The operator slides the products bearing the symbols past either window from right to left, or from left to right, or in another direction, in a “swipe” mode. Alternatively, the operator merely presents the symbols on the products to a central region of either window in a “presentation” mode. The choice depends on operator preference or on the layout of the workstation. Each symbol may be located low or high, or right to left, on the product, or anywhere in between, or on any of six sides of a box-shaped product. Each symbol may be oriented in a “picket fence” orientation in which the elongated parallel bars of the one-dimensional UPC symbol are vertical, or in a “ladder” orientation in which the symbol bars are horizontal, or at any orientation angle in between. When at least one of the scan lines from the scan pattern generator sweeps over a symbol, or when an image of the symbol is captured by the imager, the symbol is processed, decoded and read, and the product associated with the symbol is identified for further processing, e.g., price look-up or inventorying.
It is desirable to have the FOV of the imager large at a near working distance or a close proximity to either window of the workstation so that the FOV covers the entire symbol. At further working distances, it is preferred to have the FOV diverge slowly. As advantageous as the use of imagers has been, the range of working distances or ability to read symbols by the imager is limited by the imaging optics and by the number of pixels in, or resolution of, the imager. A ratio between the smallest imaged bar/space area, or module, of the symbol to the number of pixels on which the bar/space area is projected at the imager, that is, the number of pixels covered by the bar/space area, is known as “pixels per module” or PPM. When the FOV diverges rapidly, the PPM decreases rapidly too, which, in some cases, limits the capability of the imager to decode and read symbols at far working distances.
In cases when the symbol is tilted in a particular direction at a substantial imaging angle, for example, 45 degrees, to the imager, such as in the bi-optical workstation described above, the PPM is further reduced as a function of the cosine of the imaging angle due to the projection effect. Also, the tilted symbol appears to look denser than it actually is. It is desirable to increase the PPM in that particular direction to compensate for the loss of the PPM, the reduced resolution, the apparent increased density, and the decreased capability to decode and read such tilted symbols.